Microphonic material and method of producing the same



NOV. 24, 1931. H. FR 1,833,641

MICROPHONIC MATERIAL {AND METHOD OF PRODUCING THE: SAME Filed March 15,1930 i f ifi -Z f x: I? I".

hive/Mar Y H f. [/0777 Patented Nov. 24, 1931 f: STATES PATENT OFFICEJEAL E, TRUTH, or nrvnasrnnnmnors, AssIenoE To WESTERN ELECTRIC COMPANY,

NcoRToEATEn, or N W YORK, N. Y., A coRroRATIoN OF N W YORK nncnornomcMATERIAL AND METHOD OF rnonuome THE SAME.

Application fi led larch 15, 1930. Serial No. 486,175.

1 g invention relates to microphonic materia'l'n and methods ofproducing the same, v and mor'e particularlyto a material consisting'gof particles of insulating substance 5' coated" with a conducting layer,for use in microphones, telephone transmitters and repea'ters, andlikeinstruments. and to methterialof particles of an elastic substance suchas glass or other Vitreous substance, each j particle being coated withathin layer of metal,such as platinum or iridium, or other "conductingsubstance. In order to'decrease 1 the specific gravity of the materialto the point where it will compare favorably with the carbon granulescommonly used as a Inicrophonic material, the insulating substance, hasincorporated therein bubbles of airorotherf gas, which may be formed bysuitably working the insulating substance while in molten form' orsmaybeformed by mixing a gas-generating substance in the molten insulatingsubstance to form the bubbles autogenously; The invention -furthercontemplatesan ji'mroved method for producing a layerof-conductingsubstance upon the surface of the insulating substance.Suchv layer maybe formed by incorporating the conducting substance inthe insulating substance, forming the resulting material into.

particles, and subsequently reducing the ma- 40 terial by any suitabletreatment to leave a residue of conductingsubstance on the surfacesoftheparticles. The particles used as microphonic material arepreferably of spherical or spheroidal shape,although granular particlesmay also be used.

The invention will be more fully understood from the followingdescription, taken in connection with'the appended drawings, in whichFig. 1 is an enlarged cross-sectional view as illustrating the inventionrather than limiting it, the reference numeral 10 desigshowing theinsulating material having gas bubbles incorporated-therein,

Fig. 2 is an enlarged sectional view of a spherical particle having acoating of conducting substance thereon, and

Fig. 3 is an enlarged sectional view of. a granule of the same materialhaving a coating of conducting substance thereon.

In the drawings, which are to be construed nates generally a piece ofglass or other fusible insulating substance having incorporated thereinbubbles ll of air or other gas. These bubbles may be formed by mixingwith the'insulating substance a substance which, upon being raised to atemperature above the melting point of the insulating substance, willgenerate gas in quantities sufiicient to form thebubbles. In practicingsuch a process, the insulating substance and the gas generatingsubstancemay be mixed together in granular or powdered form and the mixture isthen-raised to the melting point of the insulating substance and to suchhigher temperature as maybe necessary to cause the generation of as,whereupon the material is cooled and reduced to particles of suitablesize as described below.

Another method of forming the article illustrated in Fig. 1 is toagitate molten glass in such a manner as to cause it to become seedy;that is, filled with bubbles. This may be done by alternately drawingout and folding upon itself a small portion of molten glass, causingbubbles to become incorporated therein in the manner known to thoseskilled in the art, or it may be accomplished by other agitating methodsknown in the art. The material may then be'comminuted and solidified ina shot tower in the known manner, forming globules as indicated in Fig.2, or itmay be chilled and crushed to reduce it to small particles ofgranular shape as indicated in Fig. 3. These particles may then besifted to select the granules of the proper size, granules which pass a50-mesh screen and are retained on a lUO-mesh screen being suitable formost purposes, although the size of the particles maybe widely Varieddepending upon the particular nature of the instrument in which thematerial is to be used, and upon other factors.

The coating of metal or other conducting substance 12 may be applied tothe particles by cathodic sputtering in the manner dis-.-

closed in. my copending application Serial No. 329,645, filed December31, 1928 .or it may be applied in any other suitable, nown manner. Onesatisfactory manner of applying the coating is by incorporating in themolten material a salt of, the metal, such as gold chloride, dissolvingthe salt in the molten vitreous material, then forming the mixture intoglobules or granules, and subsequently reducing the salt at-the surfaceof the globules or granules by subjecting them .to a reducing atmosphereat a high temperature. The metallic salt may also be dissolved in aglaze, which may then be applied to the globules or granules as acoating and subse- 'quently subjected to a reducing process.

Either of these treatments leaves a metallic deposit upon the globuleswhich is uniform and satisfactory for microphonic purposes. The coating12 may'also be applied by applying to the particles a liquid solution ofa compound of the metal, and then precipitating the metal therefrom uponthe surfacesof v the particles, the process being similar to thatemployed in the silvering of mirrors. It is also possible to coattheparticles by vaporizing the metal or a compound thereof and.

condensing the vapor upon the surfaces of the particles in a'mannersimilar to the well known sherardizing process. Various mechanicalmethods may also be employed, such as tumbling the articles in contactwith the 'metal to be applled thereto, or applying the v metal in theform of a paint and subsequently burning out the combustible ingredientsof the paint to leave a residue of conducting metalv The microphonicmaterial described herein may be manufactured at a relatively low cost,

' and is very elastic and durable in use.

While. the invention has been described specifically, it is'to beunderstood that various changesmay bemade without departing ing theparticles with metal.

4. A method of producing microphonic material which consists in workingmolten glass to produce air bubbles therein, forming the glass intoparticles, and coating the particles with metal.

5. A method of producing microphonic' material which consists in workingmolten glass to produce air bubbles therein, solidifyingand reducing theglass to small particles,

selectingparticles of a substantially uniform size and coatingtheselected particles with a conducting material. v

6. A method of producing microphonic material which consists inincorporating into a vitreous material a compound of an insulatingmaterial, forming the mixture into particles, and reducing the compoundat the surfaces of the particles to leave a coating of conductingmaterial thereon.

7. Microphonic material consisting ofplatinum-coated particlesv of glasshaving gas bubbles incorporated therein.

8. Microphonic material consisting of spheroidal particles of glasscoated with a noble metal and having gas bubbles incorporated therein. I9. Microphonic material consisting of particles of glass having airbubbles incorporated therein and coated with a noble metal by a cathodicsputtering process.

In witness whereof, I hereunto subscribe 28th day of February, A. D.

HAL F. FRUTH.

my name this 1930.

